In recent years, introduction of coherent optical communication using a digital signal processing technology has been and is progressing. In the coherent optical communication, the transmission rate has been raised in order to increase the transmission amount. However, in proportion to the rise of the transmission rate, waveform distortion occurs and deteriorates the transmission quality. Therefore, a technology is available by which orthogonally polarized waves of an H (Horizontal) polarized wave and a V (Vertical) polarized wave of a transmission signal (optical signal) of a transmitter are multiplexed. By the polarization multiplexing, the transmission rate per one polarized wave can be decreased to implement high speed communication.
A transmission signal (polarization multiplexed signal) having passed through a transmission line is demultiplexed into two orthogonally polarized wave components by a polarization demultiplexing circuit provided at an input terminal of a reception unit of a receiver (digital coherent receiver). The reception signal after demultiplexed successively passes several units for separation of I and Q components, photoelectric conversion and analog to digital conversion and then is inputted as a quantized digital signal to a signal processing circuit.
Since the polarization multiplexed signal is influenced by polarized wave rotation, polarization mode dispersion (PMD) and so forth in the transmission line, where only the polarization demultiplexing circuit is used, it is difficult to fully demultiplex a polarized wave. A process for polarization demultiplexing is performed by an adaptive equalization circuit provided in the signal processing circuit.
The adaptive equalization circuit is configured, for example, using a butterfly type finite impulse response (FIR) filter where a plurality of filters are coupled in a crossed coupling form. For compensation only for polarization demultiplexing, a plurality of filters for use in a butterfly type FIR filter can be configured by one tap. In contrast, in a case where equalization in regard to linear deterioration such as PMD, wavelength dispersion or the like is also compensated, a complex digital FIR filter having a plurality of taps is used for each filter. The tap number assumes various values from several to several tens taps depending upon the range of the compensation. As an algorithm for optimally controlling the coefficient of each filer, a constant modulus algorithm (CMA) method and other algorithms have been proposed (refer to, for example, C. Richard Johnson and five others, “Blind Equalization Using the Constant Modulus Criterion: A Review,” Proceedings of the IEEE, vol. 86, No. 10, October 1998, hereinafter referred to as Non-Patent Document 1).
At a succeeding stage to the adaptive equalization circuit in the signal processing circuit, a frequency offset compensation circuit is provided which compensates for a frequency offset (difference between an optical reception frequency and a frequency of local oscillation light) of the receiver. As regards the frequency offset compensation circuit, for example, a technology has been disclosed which removes coded components and noise components from complex electric field information by raising a phase-shift keying (PSK) signal of m values to the m-th power to extract a component of a frequency offset (refer to, for example, Andreas Leven and three others, “Frequency Estimation in Intradyne Reception,” IEEE Photonics Technology Letters, vol. 19, No. 6, pp. 366-368, March 2007, hereinafter referred to as Non-Patent Document 2). Further, a technology is disclosed wherein provisional decision of a signal is performed, removes coded components by subtracting a result of the provisional decision and expands an estimation range of a frequency offset (refer to, for example, Hisao Nakashima and ten others, “Novel Wide-range Frequency Offset Compensator Demonstrated with Real-time Digital Coherent Receiver,” 34th European Conference and Exhibition on Optical Communication (ECOC2008), Mo. 3, D. 4, September 2008, hereinafter referred to as Non-Patent Document 3).
Further, a technology is available wherein a difference between an current output and a previous output of an FIR filter for a PSK signal is calculated to update an equalization coefficient (refer to, for example, Japanese Laid-open Patent Publication No. 2011-211706). Another technology is available wherein a frequency offset is monitored and calculation of a coefficient of an equalizer for wavelength dispersion compensation is performed until the frequency offset becomes equal to or lower than a given value (refer to, for example, Japanese Laid-open Patent Publication No. 2010-268390). A further technology is available wherein a compensation characteristic is controlled so that a frequency difference between frequency data of a signal that has undergone wavelength deterioration of a transmission line and frequency data of a reference waveform having no wavelength deterioration is minimized (refer to, for example, Japanese Laid-open Patent Publication 2002-261692). A still further technology is available wherein a dispersion compensation amount is adjusted so that a frequency difference between a clock of a reproduced optical signal and a clock given in advance is reduced (refer to, for example, Japanese Laid-open Patent Publication NO. 2008-19071).